Conventional solar cells are fabricated by diffusion of a high concentration of phosphorous atoms into a low resistivity, 1-10 ohm cm, "p" type silicon wafer, 0.010-0.020 inches thick, cut from a Czochralski grown, single crystal, silicon ingot. The process creates a shallow N.sup.+, P junction at the top of the wafer; the non-diffused body of the wafer, below the junction, is called the base. The thick base provides the physical support and ruggedness necessary for handling. Contacts to top and back surfaces and an antireflection coating applied to the top surface complete the cell.
It is well known that a very thin, high resistivity base, back surface field, BSF, cell should have greater resistance to radiation damage and higher open circuit voltages than the conventional cell. Furthermore, a high minority carrier lifetime, very thin base, BSF cell could, theoretically, have significantly higher open circuit voltages and efficiencies under concentrated sunlight than presently available high concentration cells. These advantages are not achievable, at least not to the same extent, if the cell base is formed by epitaxial deposition because the properties of epitaxially deposited silicon are inferior to those achievable for Czochralski or float-zone silicon wafers. However, a primary problem of thin cells in the past was that minority carriers readily reached the cell back contact where they recombined instantaneously. As a result, the voltage and current and, consequently, the efficiency of thin cells was reduced, the thinner the base the greater the degradation. It is common knowledge that an efficient thin base cell must include a back surface field which acts as a barrier, diminishing minority carrier movement to and recombination at the back contact. A know method for making a back surface field barrier is to diffuse a high concentration of boron or aluminum into the back surface of the base to create a P.sup.+, P junction and an electric field. Solar cells so made are called back surface field ("BSF") cells. Thus, in the fabrication of back surface field cells, high concentrations of dopant impurities must be diffused into both the back and front surfaces. Unfortunately, high concentrations of lattice defects form in thin silicon wafers during these diffusions; the thinner the wafer, the greater the resultant defect concentration. Formation of such defects also depends upon the element being diffused and its concentration in the wafer. Diffusion generated defects degrade the cell junctions and the minority carrier lifetime throughout a very thin base cell. In corroboration of the above statements, high resistivity, high minority carrier lifetime silicon, very thin (0.002-0.004 inch base) cells made by direct diffusions into the thin wafers have had lower open circuit voltages than thick base, BSF, cells made from the same high quality silicon using identical processing. Theoretically, the results should be exactly the opposite.
It is an objective of this invention to provide a method for fabrication of very thin base cells without the necessity of forming the cell base by means of epitaxial deposition, but rather by using a high quality silicon single crystal wafer to form the base. It is also an objective of this invention to provide a method for fabricating very thin base cells which reduces diffusion created degradations. It is a further objective of this invention to provide a method for fabricating more effective barriers, greatly diminishing minority carrier movement to both the back and front surfaces of very thin base cells, thereby reducing surface recombination and increasing cell voltage. An additional objective of this invention is to provide a rugged high efficiency very thin base cell.